Although I'm sure some would appreciate very in-depth abilities to tweak gem spawns, personally, a simple multiplier would suffice.

Personally, I would use it to set the spawn rate to 0, so that jewelry is only found as rare loot in dungeons, but I'm sure others would appreciate the ability to raise or lower the quantity that is spawned in the world.

Gem spawns are not possible to adjust using mod configs.

Instead regular datapacks can do this.